Work Package 2

The lack of direct observations of ARs calls for an analysis mainly based on the model’s representation of reality, which is anyway guaranteed by the renowned expertise of the research units in the field of meteorological/hydrological modelling. Satellite observations will help in monitoring ARs offshore, especially concerning the characterization of the vertical structure. WV vertical profile will be also available through the EARLINET Lidar network and together with MW radiometers and GPS data will be used to support model simulations and process studies.

Activity 2.1 Meteorological modelling and satellite observations 

The main factors that control the amount of precipitation include moisture availability and thermodynamic profile of the flow impinging on the orography. Moist-neutral stratification of ARs offshore combined with strong low-level flow and large WV content, provides an ideal condition for significant orographic rainfall enhancement. Thus, ARs often focus heaviest precipitation on the windward slopes of mountains, where uplift is provided. The focus will be on local scale processes that determine rainfall distribution. Due to coarse resolution, reanalyses are not suitable for representing AR-orography interaction over a very complex environment. High-resolution simulations will investigate the characteristics of the main ARs identified in WP1 and CC-A, also exploiting satellite observations.

Activity 2.2 Hydrological modelling

ARs have been linked to many of the largest winter floods over the coastal areas of Europe where they cause extreme rainfall particularly in autumn and winter. High-resolution hydrological simulations will be performed in order to investigate events identified in WP1 and CC-A, in particular establishing linkages between high runoff depending on basin characteristics, landfalling ARs trajectories and orographic forcing. 

The probability and the timing of flood risk will be investigated. Using a distributed parameter hydrological model based on a regular spatial grid that includes explicit computation of the physical processes, it is possible to study the response of the basins to different ARs. 

For the case studies, in addition to the simulated flow discharge values, we will also exploit a stress index approach: discharge and runoff are combined with geographical information related to the upstream basin displacement, using other variables, such as the hydraulic radius (a linear function of the drained area) and time of concentration (that implicitly considers runoff conditions upstream). Stress indices provide information in each point of the drainage network, and their mutual variation from upstream to downstream along the river path is proportional. The methodologies will identify the relevant features of an AR conducive to high hydrological impact (e.g. intensity/duration) and watershed characteristics critical for turning precipitation into flooding, such as antecedent soil conditions, snow-rain partitioning and snow melting which are still uncharted aspects, very relevant given the complexity of the Italian territory.